Yeasts refermentation

During mead fermentation, several problems are generally encountered. For example, the anticipated alcohol content may not be achieved within the time desired. There may also be a lack of uniformity in the final product, due to differences in water content of the honey used. In some situations, such as worts with high sugar contents, successive addition of honey is needed to avoid premature termination of fermentation. This likelihood of stuck fermentation is increased as most mead is made empirically, without adjustments. This can lead to subsequent yeast refermentation and secondary fermentations by lactic and acetic acid bacteria. These can undesirably increase acidity and the production of volatile esters (Casellas, 2005). The presence of these compounds alters... 

Sweet wines, with potential for yeast refermentation, or wines with potential for malo-lactic fermentation, go through a membrane filtration prior to bottling. The membrane filters come in different porosities 0.65-p, pore size is used most commonly when 100 percent yeast removal is desired, and 0.45-p, pore size is used for malo-lactic bacteria removal. Proper sterilization of bottling equipment downstream of the membrane filter is essential to maintain the yeast- or bacteria-free nature of the wine after filtration. 

Dekoninck, T. M. L., Mertens, T., Delvaux, R, Delvaux, R R. (2013). Influence of beer characteristics on yeast refermentation performance during bottle conditioning of belgian beers. Journal of the American Society of Brewing, 7i(l), 23-34. 

Sweet white wines, however, are subject to refermentation in contact with yeast. Protection from refermentation includes a combination of three factors chilling (2°-10° C), S02 (25-35 mg/L free), and reduction of yeast population to a minimum level by pad or tight diatomaceous earth filtration. At bottling, yeasts are 100 percent removed from the wine to prevent refermentation in the bottle. 

On the other hand, it is claimed that these advantages are offset by the necessity of sterilizing the must, pressed grapes and juice. In practice, however, the benefits derived from the use of pure culture yeast can often be had without resorting to the costly operation of sterilization. The pure culture yeasts arc especially advantageous in the manufacture of white and sparkling wines and in the refermentation of the latter. They are used extensively abroad for these purposes and have secured some recognition and produced excellent results in some wineries in this country. 

Presumptive identification of a yeast suspected to be Zygosaccharomyces can be made by growing the isolate on media supplemented with 1% acetic acid. Saccharomyces sp. is not able to grow under these conditions. Further, among the species of Zygosaccharomyces, Z. bailli and Z. bisporous grow, whereas Z. rouxii will not. Other native yeast species may also be able to tolerate environmental acetic acid (Boulton et al., 1995). However, as the authors note, refermentation in sorbated sweet wine virtually eliminates these from consideration. 

Some examples of different CQPs and their specific microbiological problems along with possible corrective measures are found in Table 10.1. As an example, detection of one cell of Saccharomyces in a bottle of sweet, unfortified wine may be enough to cause refermentation in the bottle. In this case, a preventative measure that could be taken would be assurance that the bottling line was correctly sterilized and that sterile filtration equipment functioned properly. Additional measures would be inclusion of sorbates to limit potential yeast growth. Known as hurdle technology, this concept relies on implementation of several preventative measures as... 

Sulfur dioxide acts more on wine bacteria than on yeasts. Lower concentrations are conseqnently sufficient for hindering their growth or suppressing their activity. No systematic studies have been carried out on this subject but this fact is weU known and is often demonstrated in practice. For example, in the case of a red wine still containing sugar (a site for simultaneous alcoholic and lactic fermentation), a moderated sulfiting (3-5 g/hl) can initially block the two fermentations. Afterwards, a pnre alcoholic refermentation can take place withont the absolute necessity of an yeast inoculation. 

In practice, carefully adjusted sufficient concentrations must be used to avoid accidental risks. The refermentation of a sweet wine can start in the lees of a tank containing a sufficiently high yeast population to ensure the combination of the SO2. Simultaneously, at least for a certain amount of time, all of the liquid remains limpid without a refermentation, with 60 mg of free SO2 per liter. If the fermentative process begins from the lees, the refermentation seems possible in spite of the high concentration of free sulfur dioxide. 

Sorbic acid is not a winemaking tool. It does not affect the rules of mutage for sweet wiues. It is incapable of stopping fermentations that are underway. Sorbic acid is exclusively used for the conservation of sweet wines to avoid their refermentation. It can be added to wine after the elimination of yeasts by racking, centrifugation or filtration. 

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